Large aircraft, for example of the wide-body variety, typically employ multi-wheel landing gear or landing gear trucks which may be steerable to some degree when the aircraft is taxiing on the ground. The aircraft is usually provided with a steerable nose wheel or nose wheel landing gear and a plurality of main landing gear trucks adapted to cooperate with the nose wheel landing gear in executing a turn.
In a simplest case, the main landing gear comprises a pair of wheels mounted one at either side of the fuselage and rearward of the nose wheel. In this instance, when the nose wheel landing gear is steered, the rearward wheels merely follow as the aircraft turns about a point where the nose wheel transverse axis and the main wheel transverse axis intersect. For larger aircraft, individual wheels are replaced by trucks of wheels, for example comprising four wheels each. When the two main wheels are replaced with a pair of four wheel trucks, little difficulty is encountered and the trucks need not rotate about the axis of the supporting strut. However, for larger aircraft a greater number of weight bearing support wheels is required and a total of four trucks, two on each side of the aircraft, has typically been employed. In this case, two forward four wheel trucks are desirably non-steerable and substantially track the nose wheel landing gear in a turn wherein the transverse axis of the nose wheel and the central transverse axes of the two forward trucks coincide at a point about which turning takes place. However, the addition of "staggered" trucks, rearward on the aircraft from those already discussed, will bring about tire scrubbing as a turn is executed about the aforementioned point unless additional measures are taken. Therefore, the rearward trucks are steered to achieve a condition where their transverse axes also intersect the aforementioned turning point. The function of the forward and rearward trucks may be interchanged so that the forward pair of trucks is steered while the rearward trucks are stationary. As will be understood by those skilled in the art, the difference between the two situations determines the direction of steering of the trucks.
An intermediate situation relates to an aircraft having a steerable nose landing gear and a pair of six wheel trucks disposed rearwardly of the nose landing gear. In this case, the truck wheels of the main landing gear are far enough away from one another, i.e., on three separate axles, so that, unlike four wheel fixed trucks, unsatisfactory tire scrubbing occurs wherein the forward or rearward wheels tend to drag around a turn. Attempting to steer this large a truck about its support strut would serve little purpose.